Preheat oven to 350F (175C) and lightly but thoroughly grease and flour (or spray with baking spray) a 12 count donut tin. Set aside.

In a large mixing bowl, whisk together flour, sugars, cornstarch, baking powder, baking soda, and salt.

Measure out steaming water into a large measuring cup or a separate mixing bowl. Immediately add cocoa powder and whisk until smooth.

Whisk in butter until combined (it's OK if small lumps form).

Add egg and vanilla extract to cocoa mixture and whisk well.

Add buttermilk and whisk until smooth.

Add chocolate mixture to dry ingredients and use a spatula to gently fold together until about halfway combined.

Add mini chocolate chips and continue to gently stir ingredients until just combined (do not over-mix or donuts could be dense, dry, and tough).

Drop batter evenly into prepared donut molds (or pour the batter into a piping bag and pipe it in, this will give you neater donuts!).

Transfer to center rack of 350F (175C) oven and bake for 10-12 minutes or until a toothpick inserted in the thickest part of a donut comes out clean or with fudgy crumbs. Allow donuts to cool in pan for several minutes before carefully turning out to a cooling rack to cool completely before covering with glaze.

Combine chocolate chips and coconut oil in a small microwave-safe bowl and heat in 25-second intervals, stirring well in between, until chocolate and oil are completely melted and mixture is smooth.

Dip the smooth surface of each donut entirely into the chocolate glaze. Return to a cooling rack for the glaze to set before enjoying. If desired, decorate glaze with sprinkles while icing is still wet.